In an internal combustion engine with ignition control, the term “misfire phenomenon” or misfire indicates a phenomenon of poor combustion occurring within one or more cylinders; in particular, misfire is referred to as ignition misfire when it is caused by insufficient or no energy from the spark generated by the spark plug, or referred to as injection misfire when it is caused by insufficient or no fuel supply.
The existence of the misfire phenomenon proves particularly detrimental, since poor combustion involves a decline in engine performance, an increased level of polluting emissions from the engine and possible permanent damage to the catalyser. Therefore, European regulations (in particular European Directive 70/220 and successive amendments thereto) on emission limits for motor vehicles makes provision both for an effective recognition of the misfire phenomenon, and that the driver be alerted to the existence of the misfire phenomenon by means of a flashing light placed on the dashboard. More specifically, European regulations on emission limits for motor vehicles provides that an increased level of polluting emissions be notified when the number of misfires in a first time interval (for example 1000 TDC—top dead centres) exceeds a first threshold, and that permanent damage to the catalyser be notified when the number of misfires in a second time interval (for example 200 TDC—top dead centres) exceeds a second threshold.
Currently, recognition of the misfire phenomenon is carried out indirectly, that is, by analysing the instantaneous value of the angular acceleration of the drive shaft or the instantaneous value of the engine torque, since direct analysis of combustion by means of sensors placed inside every cylinder proves impracticable for reasons of cost.
The most widespread method of recognising the misfire phenomenon consists in analysing the angular acceleration of the drive shaft; in particular, for example described in patent application EP-0637738-A1, use is made of the signal supplied by a gear wheel integral with the drive shaft, also called a phonic wheel, in order to calculate the value of the angular acceleration of the drive shaft in relation to given angular positions of said drive shaft, and the misfire phenomenon is recognised if the absolute value of angular acceleration of the drive shaft is greater than a given threshold value. It is clear that any structural (ellipticity) or assembly (eccentricity) irregularities of the gear wheel must be compensated in order to prevent them from being confused with variations in speed of said gear wheel, with consequent errors in the application of control strategies. The rotational speed of the gear wheel is analysed by taking account of an angular window containing a single point of generation of engine torque, that is a single TDC; since the distribution of TDCs over the entire revolution of the gear wheel is a function of the number of the cylinders and since every tooth of the gear wheel must be compensated, it is clear that the method of compensating for the irregularities of the gear wheel must take account of the number of cylinders in the system.
Some methods have been proposed for finding the angular acceleration of the drive shaft by means of the gear wheel, which methods provide compensation for any irregularities in said gear wheel; compensation of any irregularities in the gear wheel is carried out by determining a vector of correction coefficients, each of which is algebraically added to the results of respective measurements made by reading the gear wheel. The values of the correction coefficients are determined in the course of a fuel cut-off phase, because in this way the effects of combustion are absent and no further torque is generated to disturb the measurement of the speed of the gear wheel.
However, each of the methods proposed for compensating any irregularities of the gear wheel is designed for a respective number of cylinders and therefore proves not to be applicable to an engine with a previously unknown number of cylinders. Therefore, it is necessary to produce, for each number of cylinders, a respective electronic exchange capable of recognising the misfire phenomenon, with a consequent increase in costs associated with the lack of standardisation, or else it is necessary to implement in any one electronic exchange several methods for compensating any irregularities in the gear wheel to be used as an alternative as a function of the number of cylinders of the engine, with a consequent increase in costs associated with the greater use of memory of the electronic exchange.